The synthesis of a thermoelectric Na x Co 2 O 4 /Ag composite was attempted by the polymerized complex (PC) process using AgNO 3 as an Ag source and subsequent sintering at 1153K for 72ks. The effects of the PC process and Ag addition to Na x Co 2 O 4 on microstructure and thermoelectric properties of the Na x Co 2 O 4 /Ag composite were investigated. Ag was hardly substituted for Na and Co sites, and the sintered sample was composed of the Na x Co 2 O 4 and Ag phases. The electrical resistivity of the composite was smaller than that of the Na x Co 2 O 4 single phase and the Seebeck coefficient was slightly enhanced by Ag addition, resulting in the significantly large power factor. However, most of precipitated Ag particles in the Na x Co 2 O 4 matrix were coarse, 5–8μm in size, and the thermal conductivity of the composite was high as compared to the Na x Co 2 O 4 single phase. From these results, the dimensionless figure of merit of the composite was almost the same as that of the Na x Co 2 O 4 single phase.